The present invention relates to a planar light source. More specifically this invention relates to a planar light source based on a light guiding system in which a bottom surface of a light guiding member has a light scattering function for homogeneously emitting light to the outside, and further to a method of forming a light scatterer pattern realizing a light scattering function.
A plane display lamp (with the display area of several tens cm2 or less) are used in various types of devices and equipment used indoors to display a state of or an instruction for an operation of the devices or equipment. One example of the plane display lamp is a pilot lamp in which a light source such as a compact incandescent lamp or a light-emitting diode (LED) is attached to a casing having a colored plastic plate surface. In these types of lamps, when a letter or a sign is to be displayed on a display surface, as the luminance just above the light source is higher than that in the peripheral area of the display surface, non-uniformity in luminance on the display surface is large, and the letter or sign displayed on the surface can not well be visually recognized. Further these types of display lamps allows only discrimination between two states basically by turning ON or OFF the lamp. Also there is a plane display lamp in which a light source capable of emitting light changing its color is provided under a semi-transparent plastic plate, but non-uniformity in luminance varies according to a color of emitted light more remarkably as compared to the case of single color plane display lamp, so that the adaptability to be visually recognized is further lower.
As a means for reducing non-uniformity in luminance, there has been known the method used in a planar illumination device applied as a back light for a liquid crystal display unit. In this method, a plate-like light guiding member is used as a planar light source, and a linear light source such as a fluorescent light source or a plurality of dot-like light sources such as LEDs are provided around the plate-like light guiding member. Generally a reflection layer with distributed reflectivity is provided on a bottom surface of the light guiding member so that the light sources emit light with substantially homogeneous intensity from a surface of the plate-like light guiding member.
In the pilot lamp with the conventional type of light source such as an incandescent lamp or an LED attached therein as described above, non-uniformity in luminance is large on the light-emitting surface, so that the visibility of displayed contents is low and also the display quality is disadvantageously low.
Further with the plane display lamp having a light source capable of emitting light changing its color to provide light with a desired color, it is impossible to obtain the excellent display quality.
It is an object of the invention to provide a planar light source with reduced non-uniformity in luminance by using a plate-like light guiding member.
It is another object of the invention to provide a planar light source with reduced power consumption and having a light source which can easily be driven.
It is still another object of the invention to provide a planar light source having two dot-like light sources.
It is another object of the present invention to provide a planar light source having only one dot-like light source, yet capable of reducing non-uniformity in luminance on a light-emitting surface.
It is another object to provide a method of forming a light scatterer pattern printed on a bottom surface of a plate-like light guiding member.
In a first aspect of the present invention, one dot-like light source is provided at a center of each of the opposite side faces of a rectangular plane light guiding member respectively. To reduce non-uniformity in luminance of light coming out from a surface of this light guiding member, a light scatterer pattern with reflected light intensity two-dimensionally distributed thereon is formed on a bottom surface of the light guiding member.
As the dot-like light source, one LED chip loaded on the same package or a plurality of LED chips, for instance, for three RGB elementary colors loaded adjacent to each other on the same package may be used. In the latter case, as a plurality of LEDs are provided adjacent to each other, when light for each color is emitted, non-uniformity in luminance does not substantially change, so that a plurality of states can be displayed by using lights for different colors.
What is important in the present invention is a light scatterer pattern formed on a rear side of a light-emitting surface of a light guiding member so that light emitted from a dot-like light source is taken by a plate-like light guiding member to give uniform brightness, namely uniform luminance on the light-emitting surface of the light guiding member. By giving a certain level of regularity to a pattern form, it has become possible to provide homogeneous luminance distribution on an entire light-emitting surface of a light guiding member and also to provide planar displays having apparently uniform brightness.
To realize the optimal pattern as described above, how to form a dot pattern of a light scatterer on a bottom surface of the light guiding member is important.
A light scatterer dot pattern formation method according to a second aspect of the present invention comprises the step of:
a) forming a light scatterer homogeneously all over a bottom surface of a light guiding member for assessment having the same size and made from the same materials as said light guiding member;
b) assembling said planar light source;
c) measuring luminance at each coordinate point on a light-emitting surface of said planar light source;
d) converting the measured luminance at each coordinate point through inverse-proportional operation to standardized value using the lowest value in a distribution the luminance as a reference;
e) calculating an area of a light scatterer dot at each coordinate point from the standardized value obtained from the conversion in the previous step; and
f) forming a pattern of light scatterer dots each area thereof is calculated in the previous step on a bottom surface of a light guiding member different from that for assessment.
A third aspect of the present invention provides a planar light source which can reduce non-uniformity in luminance on the light-emitting surface even when only one dot-like light source is used. In the planar light source using one dot-like light source according to the present invention, three of the four side faces of the rectangular planar light guiding member are inclined side faces. On a central one of these three inclined side faces, or on a bottom surface of the light guiding member just below the side face, one dot-like source is provided. Further provided on a bottom surface of the light guiding member just below the top surface of the light guiding member is a light scatterer dot pattern formed according to the method according to the second aspect of the present invention as described above, and further a light scatterer pattern is provided on a bottom surface of the light guiding member just below an inclined side face adjacent to an area where light from the dot-like light source does not fully reach and the luminance is sufficient.